Containers for packaging liquid products, particularly dairy products such as milk and cream, are made out of coated paper-based board. The coating may be on one side of the board but is generally on both sides and is usually polyethylene, although other water-proofing substances may be used.
To function effectively in such a container, the board must be resistant to the effects of the liquid. For liquid dairy products the most aggressive penetrating component of the liquid is generally lactic acid. The most vulnerable part of the board tends to be its cut edge.
Board manufacturers have therefore investigated ways to improve the resistance of board to edge penetration by lactic acid-containing liquids. It is known that board sized with a ketene dimer (KD) has good resistance to edge penetration by lactic acid-containing liquids.
The demand for the aseptic packaging of liquids, in particular for liquid dairy products, makes it necessary to sterilize the package as well as its contents. Containers made out of board are usually sterilized by contact with hydrogen peroxide solutions at elevated temperatures (for instance, at about 70.degree. C.). Unfortunately board sized with a KD has low resistance to edge penetration by hot hydrogen peroxide-containing solutions, and of course sizing with a cellulose-reactive size like KD has the inherent economic disadvantage that it does not take place "on-machine"; in other words, it requires an aging period for the sizing to be fully effective.
It is known that board sized with a cationic rosin size (CRS) or with a conventional anionic rosin size (emulsion, paste or soap) has good resistance to edge penetration by hot hydrogen peroxide-containing liquids, but low resistance to edge penetration by lactic acid-containing liquids. There has also been a trend to make board from secondary fibre under neutral rather than acid conditions. This avoids some of the problems associated with recycled chalk fillers, and it is well known that sizing with KD's must be carried out at neutral or slightly alkaline pH's (between 7 and 8.5) in order to achieve effective sizing, while sizing paper with rosin must be carried out at pH's below 5.
U.S. Pat. No. 4,859,244 of International Paper Company discloses paper sizing agents composed of blends of fatty acid anhydrides and alkyl ketene dimers, that improve resistance to wicking on paperboard containers.
British Patent Specification 1,402,196 discloses aqueous dispersions of wax using fused or dissolved waxes and a thermosettable cationic resin as a dispersing agent, for use in sizing paper.
U.S. Pat. No. 3,922,243 of Hercules Incorporated discloses paper sizing agents composed of aqueous suspensions or dispersions of wax blends for use in the sizing of paper and containing from 99% to 93% of either a petroleum wax or a synthetic hydrocarbon wax and from 1% to about 7% of a C.sub.18 to C.sub.24 saturated fatty acid, or a blend containing from about 99.5% to about 75% of a petroleum wax or a synthetic hydrocarbon wax and from 0.5% to about 25% of an alkyl ketene dimer, or a mixture of the two kinds of blends. The sizing effect is measured by a surface sizing test, and the problems of edge penetration by lactic acid and hot hydrogen peroxide in the manufacture of liquid packaging board are not addressed.
None of the above references suggest the problem caused by sterilization by hydrogen peroxide, nor is there any indication that the sizing agents disclosed would have any effect on resistance to edge penetration by hydrogen peroxide-containing liquids nor by liquids containing lactic acid following contact with hot hydrogen peroxide solutions.
U.S. Pat. No. 4,927,496 to Hercules Incorporated addresses the problems of the penetration by hot hydrogen peroxide at the cut edge of liquid packaging board by the use of mixtures of a cellulose-reactive sizing agent (an alkyl ketene dimer emulsified with a cationic starch derivative), and a cationic rosin size mixture containing a fortified rosin, an insolubilizing agent, and a polyamide cationic resin as a conventional dispersing agent for the rosin. It does not disclose the use of combinations of ketene dimer and other sizing agents that do not contain a CRS size, as a solution to edge penetration by hot hydrogen peroxide.
There is therefore a need for method of producing a packaging board that has good resistance to edge penetration by both lactic acid and hot peroxide-containing liquids, and that provides sizing while the board is still on the paper machine, and therefore avoid traditional CRS sizes with insolubilizing agents.